Bath for and method of coating magnesium and its alloys



Patented Apr. 19, 1938 UNITED s rA'rss BATH AND METHOD OF COATING NESIUM AND ITS ALLOYS A 2,114,134 PATENT OFFICE,

Robert W. Buzzard, Kenaington. m

No Drawing.

Application March 30, 1985,

Serial No. 13,911

scu

(Granted under the act of March 3,. 1883, as amended April 80, 1928; 3'10 0. G. 757) should be first cleaned of any foreign material, v

by various known means, such as a wire brush, etching, an organic solvent, steam, etc. If the chosen cleaning medium is acid, this may be done with dilute solutions of such acids as nitric, sulphuric or hydrochloric, or it may be effected in concentrated solutions of chromic acid.

The bath consists essentially of solutions of phosphates and chromates. The color and other characteristics of the film may be varied by changing the composition of the bath. This composition is also varied to suit the particular alloy to be coated if the most satisfactory results are obtained. The rate of formation of the film depends on the composition and temperature of the bath and upon the current density used.

It is one of the valuable features of the present invention that it is not limited to any particular composition of bath or to any restricted range of temperature or current density. In most baths a reasonably good film may be produced at any temperature or with any current density normally used but there is an optimum temperature range for each alloy within which superior films are formed, which range is lowered and broadened by increasing the concentration of the-solution. In dilute solutions the temperature is preferably above room temperature, whereas in concentr-ated solutions equally good films will be produced at room temperatures. The current density determines the time required for formation of the film.

The color of the film when.direct current is ,used ranges from gray or gray-black to deep black or iridescent purple-black, the darker films resulting from the use of more concentrated solutions, lower pI-Is, alloys high in aluminum, high chrom-ate ratios and the presence of trl-valent chromium in the bath. The film probably consists of chromium magnesium and phosphate compounds and contains retained bath solution. When alternating current is used and both electrodes are made of the material to be coated, the films are a light gray.

Practically any concentration of chromates and phosphates has beenfound satisfactory, the useful concentration apparently being limited only by the solubility of the salts used, although some concentrations are preferable for particular alloys and for particular types of films. The character of the coating may be varied by changing the ratio of chromate to phosphate or the concentration of the bath. When the baths of this invention are used for dipping a gray film is produced.

The following are some proportions that have been used with very satisfactory results, it being understood that in each case the quantity of wa ter used is determined by the desired concentration:

(a) Chromic acid parts 200 Mixed chromates ;do 200 Trisodium phosphate do 100 Oxalic acid grams 100 Dibasic sodium phosphate do 200 (b) 1 per cent each of chromic acid, sodium dichromate, potassium dichromate, and oxalic acid 2 per cent each of trisodium phosphate and dibasic sodium phosphate While the above are given as examples, it has been found that baths including almost any combination of chromic acid, trivalent chromium, dibaslc or polybasic organic acid chromates, phosphates or phosphoric acid in similar chemical ratios will operate successfully.

The preferred method of preparing these baths is as follows:

The chromic acid is added to the water followed by the chromates and certain phosphates and then allowed to react completely with dibasic or polybasic organic acid, followed by the addition of dibasic sodium phosphate.

As further examples of compositions which will give anodic coatings, the following are cited:

, Per cent Chromic acid 1 Dibasic sodium phosphate 5 (d) Mono-sodium phosphate 3 Mixed chromates 3 (e) Dibasic sodium phosphate 3 Mixed chromates 3 Chrornic acid. 3

m Chromic acid 1 Mixed chromates 2 Trisodium phosphate 2 Disodium phosphate 2 (g) Chromic acid 1 Oxalic acid 1 Mixed chromates 1 Trisodium phosphate 2 After the material, which is to be coated, has

been cleaned it is connected incircuit as an electrode in a bath such as has been above defined. The current is turned on and a coat may be formed in from two minutes up, depending on the concentration and temperature of the bath and the current density. In general, five minutes has been found to be a very good working time, although satisfactory films have been applied in two minutes orless, and also in excess of an hour. The current density may vary from two or three amperes per square foot to 100 amperes per square foot or more. the limit thereof depending upon the equipment. Under certain conditions as the coating forms the current will gradually drop off. Further running from this point causes a heavy film to form on the magnesium and on continued running a correctly operated bath approaches zero current. Under other conditions good coatings may be formed with very little,change in current density during the cycle. The best concentration for any given material is readily determined by trial runs.

In addition to the baths above specified, successful runs have been made in more simple baths by using chromic acid and phosphoric acid, with or without sodium or like phosphates with chromic acid or salts of the same; phosphates with dichromates; and phosphates and trivalent -chromium compounds such as chromium chromate and chromium acetate. Some proportions of the above may be specifically mentioned:

Gms/liter Sodium mono-basic phosphate 75-1000 Sodium dichromate 25-1000 which were found most satisfactory at to C. or again 0.5 N phosphoric acid and 0.05 N chromic acid. As a variation of the last bat the following was successfully tested:

.75 N prosphoric acid .075 N chromic acid .25 N sodium hydroxide maximum limit of concentration appears to be determined by solubility factors only. The optimum bath is sludge free but very good results have been obtained in baths heavy with sludge. The solution may either be washed from the finished coating or allowed to dry thereon. The films so produced are very satisfactory as bases for other surface coatings, such as greases, oils, paints, etc. The chemically basic substances used, which may include ammonium compounds, are for regulating the pH, which appears to be the controlling factor in the formation of sludge.

It is preferable to operate without sludge on the acid side. The bath functions best in slightly acid pHs. Highly acid baths do not give good films, and if they are too basic, a heavy sludge forms.

There is evidence that the coating formed by the present method results from both anodic oxidation and also deposition. The simplest bath for practicing the present invention, therefore, contains an anode depositing ion or a reacting ion as a phosphate and an oxidizing ion as a chromate with the pH adjusted on the acid side to hold the ingredients in solution.

The invention herein described'may be manufactured and used by.or for the Government of the United States of America for governmental purposes, without the payment of any royalities thereon.

I claim:

1. A'method of forming seating on magnesium and magnesium alloys, which comprises passing electric current therethrough while the said material is functioning as an electrode in a bath containing the phosphate and chromate radicals, the pH being on the acid side and,

the current density being from 2 to amperes per square foot.

2. A method of forming a coating on magnesium and magnesium alloys, which comprises passing unidirectional electric current therethrough while the said material is functioning as an anode in a bath containing the phosphate and chromate radicals, the pH being on the acid side.

3. A method of forming a coating on magnesium and magnesium alloys, which comprises passing electric current therethrough while the said material is functioning as an electrode in a bath containing the phosphate and chromate radicals and a member of an alkali group, the pH being on the acid side and, the current density being from 2 to 100 amperes per square foot.

4. A method of forming a coating on magnesium and its alloys, which comprises passing electric current therethrough while the said material is functioning as an electrode in a bath containing the phosphate and chromate radicals, the said radicals being present in the bath in quantities up to the maximum quantities thereof soluble in the bath, the pH being on the acid side, the current density being from 2 to 100 amperes per square foot.

5. A method of coating magnesium and its alloys which comprises utilizing said material as an anode in a bath that contains any concentration of a dichromate and a phosphate up to the saturation point and passing therethrough electric current having a current density from 2 to 100 amperes per square foot.

6. A method of forming protective coatings on magnesium and its alloys, which comprises subjecting the said material to anodic action in a bath containing a phosphate and a chromate radical at a current density between 2 and 100 amperes per square foot, and permitting the adherent solution to dry thereon after removal of the material from the bath, the pH of said bath being on the acid side.

7. A method of forming a coating on magnesium and magnesium alloys, which comprises passing electric current therethrough while the said material is functioning as an electrode in a bath containing the phosphate and chromate radicals, the pH of said bath being on the acid side.

8. A method of forming a coating on magnesium and magnesium alloys, which comprises passing electric current therethrough while the said material is functioning as an electrode in a bath containing the phosphate and chromate radicals and an alkali group, the pH of said bath being on the acid side.

9. A method of anodizing magnesium and its alloys, which comprises passing electric current therethrough while the said material is functioning as an electrode in a bath containing phosphoric acid, chromic acid and a substance giving a basic reaction, the pH of said bath being on the acid side.

ROBERT W. BUZZARD. 

